Carnotaurus was a lightly built, bipedal predator, measuring 8 to 9 m (26 to 30 ft) in length and weighing at least 1.35 metric tons (1.33 long tons; 1.49 short tons). As a theropod, Carnotaurus was highly specialized and distinctive. It had thick horns above the eyes, a feature unseen in all other carnivorous dinosaurs, and a very deep skull sitting on a muscular neck. Carnotaurus was further characterized by small, vestigial forelimbs and long and slender hindlimbs. The skeleton is preserved with extensive skin impressions, showing a mosaic of small, non-overlapping scales measuring approximately 5 mm in diameter. The mosaic was interrupted by large bumps that lined the sides of the animal, and there are no hints of feathers.

The distinctive horns and the muscular neck may have been used in fighting conspecifics. According to separate studies, rivaling individuals may have combated each other with quick head blows, by slow pushes with the upper sides of their skulls, or by ramming each other head-on, using their horns as shock absorbers. The feeding habits of Carnotaurus remain unclear: some studies suggest the animal was able to hunt down very large prey such as sauropods, while other studies find it preyed mainly on relatively small animals. Carnotaurus was well adapted for running and was possibly one of the fastest large theropods.

Carnotaurus was a large but lightly built predator.[1] The only known individual was about 8 to 9 metres (26 to 30 ft) in length,[A] making Carnotaurus one of the largest abelisaurids.[B] Only Ekrixinatosaurus and possibly Abelisaurus may have been similar or larger in size, though the incomplete remains of these genera make size estimations imprecise.[C][D][E] Its mass is estimated to have been 1,350 kg (1.33 long tons; 1.49 short tons)[F] 1,500 kg (1.5 long tons; 1.7 short tons)[G] and 2,100 kg (2.1 long tons; 2.3 short tons)[H] in separate studies that used different estimation methods. Carnotaurus was a highly specialized theropod, as seen especially in characteristics of the skull, the vertebrae and the forelimbs.[I] The pelvis and hindlimbs, on the other hand, remained relatively conservative, resembling those of the more basalCeratosaurus. Both the pelvis and hindlimb bones were long and slender. The left thigh bone of the individual measures 103 cm in length, but shows an average diameter of only 11 cm.[J]

The skull, measuring 59.6 cm (23.5 in) in length, was proportionally shorter and deeper than in any other large carnivorous dinosaur.[K][L] The snout was broad, unlike the tapering snout seen in more basal theropods like Ceratosaurus, and the jaws were curved upwards.[9] As in other abelisaurids, the facial bones, especially the nasal bones, were sculptured with numerous small holes and spikes.[M] In life, a wrinkled and possibly keratinousskin would have covered these bones.[N] A prominent pair of horns protruded obliquely above the eyes. These horns, formed by the frontal bones,[O] were thick, flattened on their upper sides, and measured 15 cm (5.9 in) in length.[10] In life, they would probably have formed the bony cores of much longer keratinous horns.[P] The proportionally small eyes[11] were situated in the upper part of the keyhole shaped orbita (eye sockets).[Q] The upper part was slightly rotated forward, probably permitting some degree of binocular vision.[R]

The teeth were long and slender,[11] as opposed to the usually very short teeth seen in other abelisaurids.[9] On each side of the upper jaws there were four premaxillary and twelve maxillary teeth,[S] while the lower jaws were equipped with 15 dentary teeth per side.[T] In contrast to the robust-looking skull, the lower jaw was shallow and weakly constructed, with the dentary (the foremost jaw bone) connected to the hindmost jaw bones by only two contact points.[11][U] The lower jaw was found with hyoid bones, in the position they would be in if the animal was alive. These slender bones, supporting the tongue musculature and several other muscles, are rarely found in dinosaurs because they are not connected to other bones and therefore get lost easily.[V][12]

Sixth tail vertebra of the holotype in A) side, B) front and C) top views. Arrows show the highly modified caudal ribs.

The vertebral column consisted of 10 cervical (neck), 12 dorsal, 6 fused sacral[W] and an unknown number of caudal (tail) vertebrae.[5] The neck was nearly straight, rather than having the S-curve seen in other theropods, and also unusually wide, especially towards its base.[13] The top of the neck's spinal column featured a double row of enlarged, upwardly directed bony processes called epipophyses, making the neck flat on top. These processes were the highest points of the spine, towering above the unusually low spinous processes.[5][12] The epipophyses probably provided attachment areas for a markedly strong neck musculature.[X] A similar double row was also present in the tail, formed there by highly modified caudal ribs protruding in a V-shape, resulting in a flattened top of the tail. The end of each caudal rib was furnished with a forward projecting expansion that connected to the caudal rib of the preceding vertebra.[12][14]

The forelimbs were proportionally shorter than in any other large carnivorous dinosaurs, including Tyrannosaurus.[Y] The forearm was only a quarter the size of the upper arm. There were no carpalia in the hand, so that the metacarpals articulated directly with the forearm.[15] The hand showed four basic digits,[5] though apparently only the middle two of these ended in finger bones, while the fourth consisted of a single splint-like metacarpal that may have represented an external 'spur'. The fingers themselves were fused and immobile, and may have lacked claws.[16]Carnotaurus differed from all other abelisaurids in having proportionally shorter and more robust forelimbs, and in having the fourth, splint-like metacarpal as the longest bone in the hand.[15] A 2009 study suggests that the arms were vestigial in abelisaurids, because nerve fibers responsible for stimulus transmission were reduced to an extent seen in today's emus and kiwis, which also have vestigial forelimbs.[17]

Carnotaurus was the first theropod dinosaur discovered with comprehensive fossil skin impressions.[18] These impressions, found beneath the skeleton's right side, come from different body parts, including the lower jaw,[18] the front of the neck, the shoulder girdle, and the rib cage.[Z] The largest patch of skin corresponds to the anterior part of the tail.[AA] Originally, the right side of the skull also was covered with large patches of skin—this was not recognized when the skull was prepared, and these patches were accidentally destroyed. Still, the surface texture of much of the right side of the skull is very different from that of the left side, and probably shows some features of the scalation pattern of the head.[18]

The skin was built up of a mosaic of polygonal, non-overlapping scales measuring approximately 5 mm (0.20 in) in diameter. This mosaic was divided by thin, parallel grooves.[AB] Scalation was similar across different body parts with the exception of the head, which apparently showed a different, irregular pattern of scales.[AC][19] There is no evidence of feathers.[18] Uniquely for theropods, there were large knob-like bumps running along the sides of the neck, back and tail in irregular rows. Each bump showed a low ridge and measured 4 to 5 cm (1.6 to 2.0 in) in diameter. They were set 8 to 10 cm (3.1 to 3.9 in) apart from each other and became larger towards the animal's top. The bumps probably represent clusters of condensed scutes, similar to those seen on the soft frill running along the body midline in hadrosaurid ("duck-billed") dinosaurs.[AD][18] Stephen Czerkas (1997) suggested that these structures may have protected the animal's sides while fighting members of the same species (conspecifics) and other theropods, arguing that similar structures can be found on the neck of the modern iguana where they provide limited protection in combat.[18]

Carnotaurus is one of the best-understood genera of the Abelisauridae, a family of large theropods restricted to the ancient southern supercontinentGondwana. Abelisaurids were the dominant predators in the Late Cretaceous of Gondwana, replacing the carcharodontosaurids and occupying the ecological niche filled by the tyrannosaurids in the northern continents.[1] Several notable traits that evolved within this family, including shortening of the skull and arms as well as peculiarities in the cervical and caudal vertebrae, were more pronounced in Carnotaurus than in any other abelisaurid.[AE][AF][14]

Though relationships within the Abelisauridae are debated, Carnotaurus is consistently shown to be one of the most derived members of the family by cladistical analyses.[AG] Its nearest relative may have been either Aucasaurus[20][21][22] or Majungasaurus;[23][24][25] this ambiguity is largely due to the incompleteness of the Aucasaurus skull material.[AH][AI] A recent review suggests that Carnotaurus was not closely related with either Aucasaurus or Majungasaurus, and instead proposed Ilokelesia as its sister taxon.[AJ]

Carnotaurus is eponymous for two subgroups of the Abelisauridae: the Carnotaurinae and the Carnotaurini. Paleontologists do not universally accept these groups. The Carnotaurinae was defined to include all derived abelisaurids with the exclusion of Abelisaurus, which is considered a basal member in most studies.[26] However, a 2008 review suggested that Abelisaurus was a derived abelisaurid instead.[AK] Carnotaurini was proposed to name the clade formed by Carnotaurus and Aucasaurus;[21] only those paleontologists who consider Aucasaurus as the nearest relative of Carnotaurus use this group.[27]

The only skeleton (holotype MACN-CH 894) was unearthed in 1984 by an expedition led by Argentinian paleontologist José Bonaparte.[AL] This expedition also recovered the peculiar spiny sauropodAmargasaurus.[28] It was the eighth expedition within the project named "Jurassic and Cretaceous Terrestrial Vertebrates of South America", which started in 1976 and which was sponsored by the National Geographic Society.[28][AM] The skeleton is well-preserved and articulated (still connected together), with only the posterior two thirds of the tail, much of the lower leg, and the hind feet being destroyed by weathering.[AN][29] During fossilization, the skull and especially the muzzle were crushed laterally, while the premaxilla were pushed upwards onto the nasal bones. As a result, the upward curvature of the upper jaw is artificially exaggerated in the holotype.[AO] The skeleton belonged to an adult individual, as indicated by the fused sutures in the braincase.[10] It was found lying on its right side, showing a typical death pose with the neck bent back over the torso.[18] Unusually, it is preserved with extensive skin impressions.[AP] In view of the significance of these impressions, a second expedition was started to reinvestigate the original excavation site, leading to the recovery of several additional skin patches.[18]

The skeleton was collected on a farm named "Pocho Sastre" near Bajada Moreno in the Telsen Department of Chubut Province, Argentina.[29] Because it was embedded in a large hematiteconcretion, a very hard kind of rock, preparation was complicated and progressed slowly.[11][29] In 1985, Bonaparte published a note presenting Carnotaurus sastrei as a new genus and species and briefly describing the skull and lower jaw.[29] The generic name (Latincarno [carnis] – "flesh" and taurus – "bull") refers to the bull-like horns,[30] while the specific namesastrei honors Angel Sastre, the owner of the ranch where the skeleton was found.[31] A comprehensive description of the whole skeleton followed in 1990.[5] After Abelisaurus, Carnotaurus was the second member of the family Abelisauridae that was discovered.[32] For years, it was by far the best-understood member of its family, and also the best-understood theropod from the Southern Hemisphere.[33][34] It was not until the 21st century that similar well-preserved abelisaurids were described, including Aucasaurus, Majungasaurus and Skorpiovenator, allowing scientists to re-evaluate certain aspects of the anatomy of Carnotaurus.[AQ] The holotype skeleton is displayed in the Argentine Museum of Natural Sciences, Bernardino Rivadavia;[AR] replicas can be seen in this and other museums around the world.[19] Sculptors Stephen and Sylvia Czerkas manufactured a life-sized sculpture of Carnotaurus that is now on display in the Natural History Museum of Los Angeles County. This sculpture, ordered by the museum during the mid-1980s, is probably the first life restoration of a theropod showing accurate skin.[18][35]

Originally, the rocks in which Carnotaurus was found were assigned to the upper part of the Gorro Frigio Formation, which was considered to be approximately 100 million years old (Albian or Cenomanian stage).[29][AS] Later they were realized to pertain to the much younger La Colonia Formation,[33] dating 72 to 69.9 million years ago (Late Cretaceous, Lower Maastrichtian stage).[AT] Thus, Carnotaurus was the latest South American abelisaurid known.[14] By the Late Cretaceous, South America was already isolated from both Africa and North America.[36]

Carnotaurus is the only known carnivorous bipedal animal with a pair of horns on the frontal bone.[43] The use of these horns is not entirely clear; several interpretations have revolved around use in fighting conspecifics, in display, or in killing prey.

Greg Paul (1988) proposed that the horns were butting weapons and that the small orbita would have minimized the possibility of hurting the eyes while fighting.[11] Gerardo Mazzetta and colleagues (1998) suggested that Carnotaurus used its horns in a way similar to rams. They calculated that the neck musculature was strong enough to absorb the force of two individuals colliding with their heads frontally at a speed of 5.7 m/s each.[6] Fernando Novas (2009) interpreted several skeletal features as adaptations for delivering blows with the head.[AU] He suggested that the shortness of the skull might have made head movements quicker by reducing the moment of inertia, while the muscular neck would have allowed strong head blows. He also noted an enhanced rigidity and strength of the spinal column that may have evolved to withstand shocks conducted by the head and neck.[AV]

Other studies suggest that rivaling Carnotaurus did not deliver rapid head blows, but pushed slowly against each other with the upper sides of their skulls.[43][44] Thus, the horns may have been a device for the distribution of compression forces without damage to the brain.[43] This is supported by the flattened upper sides of the horns, the strongly fused bones of the top of the skull, and the inability of the skull to survive rapid head blows.[43]

Gerardo Mazzetta and colleagues (1998) propose that the horns might also have been used to injure or kill small prey. Though horn cores are blunt, they may have had a similar form to modern bovid horns if there was a keratinous covering. However, this would be the only reported example of horns being used as hunting weapons in animals.[6]

Analysis of the jaw design of Carnotaurus by Mazzetta and colleagues (1998, 2004, 2009) suggests that the animal was capable of quick bites, but not strong ones.[6][7][43] Quick bites are more important than strong bites when capturing small prey, as shown by studies of modern-day crocodiles.[43] Furthermore, these researchers noted a high degree of flexibility (kinesis) within the skull and especially the lower jaw, somewhat similar to modern snakes. Elasticity of the jaw would have allowed Carnotaurus to swallow small prey items whole.[43] In addition, the front part of the lower jaw was hinged, and thus able to move up and down. When pressed downwards, the teeth would have projected forward, allowing Carnotaurus to spike small prey items; when the teeth were curved upwards, the now backward projecting teeth would have hindered the caught prey from escaping.[6] Mazzetta and colleagues also found that the skull was able to withstand forces that appear when tugging on large prey items.[43]Carnotaurus may therefore have fed mainly on relatively small prey, but also was able to hunt large dinosaurs.[43]

This interpretation was questioned by François Therrien and colleagues (2005), who found that the biting force of Carnotaurus was twice as high as that of the American alligator, which may have the strongest bite of any living tetrapod. These researchers also noted analogies with modern Komodo dragons: the flexural strength of the lower jaw decreases towards the tip linearly, indicating that the jaws were not suited for high precision catching of small prey but for delivering slashing wounds to weaken big prey. As a consequence, according to this study, Carnotaurus must have mainly preyed upon large animals, possibly by ambush.[45]

Robert Bakker (1998) found that Carnotaurus mainly fed upon very large prey, especially sauropods. As he noted, several adaptations of the skull—the short snout, the relatively small teeth and the strong back of the skull (occiput)—had independently evolved in Allosaurus. These features suggest that the upper jaw was used like a serrated club to inflict wounds; big sauropods would have been weakened by repeated attacks.[46]

Cross section through the tail of Carnotaurus, showing the enlarged caudofemoralis muscle and the V-shaped caudal ribs

Mazzetta and colleagues (1998, 1999) presumed that Carnotaurus was a swift runner, arguing that the thigh bone was adapted to withstand high bending moments while running. The ability of an animal's leg to withstand those forces limits its top speed. The running adaptations of Carnotaurus would have been better than those of a human, although not nearly as good as those of an ostrich.[AW][47]

In dinosaurs, the most important locomotor muscle was located in the tail. This muscle, called caudofemoralis, attaches to the fourth trochanter, a prominent ridge on the thigh bone, and pulls the thigh bone backwards when contracted. Scott Persons and Phil Currie (2011) note that in the tail vertebrae of Carnotaurus the caudal ribs did not protrude horizontally ("T-shaped"), but were angled against the vertical axis of the vertebrae, forming a "V". This would have provided additional space for a caudofemoralis muscle larger than in any other theropod—the muscle mass was calculated at 111 to 137 kilograms (245 to 302 lb) per leg. Therefore, Carnotaurus could have been one of the fastest large theropods.[14] While the caudofemoralis muscle was enlarged, the epaxial muscles situated above the caudal ribs would have been proportionally smaller. These muscles, called the longissimus and spinalis muscle, were responsible for tail movement and stability. To maintain tail stability in spite of reduction of these muscles, the caudal ribs bear forward projecting processes interlocking the vertebrae with each other and with the pelvis, stiffening the tail. As a consequence, the ability to make tight turns would have been diminished, because the hip and tail had to be turned simultaneously, unlike in other theropods.[14]